For determining physical or chemical process variables, such as e.g. a fill level in a container, a volume, or mass, flow rate through a section of pipeline, a pressure in a line or container, a temperature of a medium, etc., a variety of types of sensors are used, which naturally are based on many different physical measuring principles.
Thus, for example, the fill level of a fill substance in a container is usually obtained via the transit time of ultrasonic waves or electromagnetic waves, particularly microwaves, which are reflected on the surface of the fill substance. When microwaves are used, these are radiated either freely into the container in the direction of the surface of the fill substance, or the microwaves are guided along a conductive element in the container. Furthermore, capacitive and radiometric measuring methods are also often used for fill level measurement.
For limit level detection, preferably the resonance frequency of an oscillating tine, or of an oscillatable structure composed of multiple oscillating tines, is evaluated. In this measuring method, the effect is utilized in which the resonance frequency differs depending on whether the oscillating tine executes its oscillations freely or, instead, in contact with the fill substance.
However differently the individual measuring apparatuses for determining the fill level or other physical variable, for example flow rate, may be constructed, one thing is common to all—they require energy. This energy is normally supplied via electrical lines.
The disadvantage of known measuring apparatuses of the type described is, among other things, that until now the energy is normally supplied from a remote, e.g. grid-fed power source, via an appropriate field bus system, with the supplied energy needing to be re-formed multiple times, e.g. with respect to the voltage level. Furthermore, the wiring required for supplying the energy usually necessitates relatively high installation costs, with the cable alone possibly incurring considerable expenses.
In DE 100 37 911 A1, DE 201 07 117 U1, or also DE 199 29 343, apparatuses for determining and/or monitoring a physical or chemical variable have already been proposed, which apparatuses seek to resolve the above-named disadvantages. Each of the apparatuses includes a sensor part, an electronics part, and a housing. At least the electronics part is arranged in the housing.
Furthermore, at least one fuel cell, e.g. to be fueled with hydrogen as the fuel, is provided, through which the energy, or, stated alternatively, power, requirement of the apparatus is at least partially satisfied. As a result of this embodiment, a measuring device supplied with energy directly in the field is provided, which can be placed at any location in the process, and there, if necessary, can be operated completely self-sufficiently, that is, without an external energy supply.
However, found to be problems in the case of these solutions are: The currently possibly inadequate ability of such fuel cells to supply power (especially a still unsatisfactory power-to-weight ratio); and, above all, the very complex and, in areas where there is danger of explosion, even critical replenishing of the often highly explosive fuel into the fuel cells of the individual field devices. Furthermore, the power output of such fuel cells is affected to no small degree by the climatic conditions of the space where they are installed, however especially also by the surrounding temperature.